The invention relates to a piezo electric drive, particularly for relatively high drive forces or, respectively, drive power.
Such a piezo-electric drive may be used as a rotary drive wherein a relative rotational movement between a reference part and a drive part is generated as well as a linear drive wherein a linear or quasi-linear displacement movement between a reference part and an output part is generated.
The main field of use of the invention is its use as a control drive wherein high operating forces must be generated for example for the actuation of vehicle brakes.
The basic concept of such a piezo electric drive is disclosed, for example, in DE Gebrauchsmuster 94 19 802.
The basic design features of such a piezo electric actuator comprises a number of piezo packets in the form of stacks of piezo elements, which are arranged between a reference part and a drive part in such a way that the height of the stack of the piezo element extends normal to the surface of the drive part and to the direction of the relative movement between the reference part and the drive part. These stacks of piezo elements are adapted to provide selectively a clamping force between the reference part and the drive part and will therefore below be called clamping piezos. These clamping piezos are supported with one end (the term end refers to the piezo element stack arrangement) on the reference part and engage, with the other end, the drive part without being connected thereto. The orientation of the row in which these clamping piezos are arranged next to each other extends transverse to the relative movement between the drive part and the reference part.
Additional stacks of piezo elements in the form of piezo packets, which, because of their function will below be called stepping piezos, are provided so that they are oriented in the stack height direction about normal to the stack height direction of the clamping piezos and in the direction of the relative movement between the reference part and the drive part. These stepping piezos are also connected with one of their ends to the reference part and with their other end to that end of a clamping piezo which engages the drive part or to a sub-group of clamping piezos.
There are two groups of clamping piezos and two groups of stepping piezos. They are electrically controlled in such a way that, at a time, one group of clamping piezos is activated, that is, electrically energized so as to be expanded and this clamping piezos then engage the drive part in a clamping fashion while the other group of clamping piezos is de-activated, that is, in a release position, in which they do not engage the drive part. Then, the group of stepping piezos associated with the clamping piezo group, which is in a clamping state, is activated so that the respective stepping piezos expand and move thereby the ends of the clamping piezos, which are in engagement with the drive part, a certain distance in the direction of movement of the drive part. As a result, the drive part is advanced by a corresponding distance relative to the clamping piezos of the other clamping piezo group, which are in a release position. Then the control is switched over that is the clamping piezos which were previously engaged are now released as well as the associated stepping piezos so that the earlier clamping piezos return from their clamping position to their original positions whereas, at the same time, the other group of clamping piezos which were previously in the release position are activated and switched to the clamping position. Then also their stepping piezos are activated. In this way, the two groups of clamping piezos generate together with the respective associated stepping piezos by their alternating activation a continuous series of steps whereby the drive part is moved in a stepwise fashion. Since the control of the piezos is in the frequency range of several kilohertz, for example 25 kHz, a quasi-continuous movement of the drive part is obtained relative to the reference part.
The presentation above represents the theory of the operating mechanism. The design principles of a piezoelectric drive as described above however has been found to be not yet practically usable.
The movements obtained in this way are extremely small. The available expansion length of a piezo packet is about 1% of the stack height. With a stack height of the clamping piezos of about 10 mm, as a result, an expansion length of about 10 μm can be achieved. On the other hand, the tolerances of the mechanical components of the drive even with high-precision manufacturing techniques are in the range of hundreds of a millimeter. Also, the stack height of the piezos is subject to manufacturing tolerances so that, in a row of piezos arranged in side-by-side relationship, all the piezo stacks have a different height within these tolerances. It has to be taken into consideration that the expansion length of the clamping piezos, assuming a range of about 10 μm, is in the roughness-depth range of mechanically worked surfaces of work pieces. As a result, the arrangement requires that the clamping piezos are held between the reference part and the drive part under tension, to be operative, since otherwise, with the available expansion length, the necessary clamping force and consequently, the needed friction force for moving the drive part cannot be achieved. Of course, the clamping piezos do not need to engage the drive part directly but they may engage the drive parts by way of an engagement shoe.
It is the object of the invention to provide an operative arrangement of a piezoelectric drive of the type described above.